The heart is under dual sympathetic and parasympathetic neural control, but noninvasive techniques to quantitatively describe the degree of control by the two individual branches of the autonomic system do not presently exist. It is becoming increasingly evident that the balance of sympathetic and parasympathetic controls influences the sensitivity of the heart to ventricular fibrillation, and that a variety of cardiac diseases are associated with abnormal autonomic control mechanisms. It thus appears desirable to develop noninvasive methods to assess the degree of autonomic control in clinical situations, with the expectation that such methods will yield diagnostic or predictive information. The proposed project is based on the results of a pilot study showing that respiratory variations in heart period correlate linearly with the degree of parasympathetic chronotropic control in chloralose-anesthetized dogs. The objective of the project is to determine under what physiological circumstances the degree of respiratory sinus arrhythmia is a noninvasive indicator of parasympathetic cardiac control in dogs anesthetized with agents in human surgery, and in the conscious, unanesthetized animal. The degree of chronotropic parasympathetic control will be assessed as the change in heart period, corrected for changes in both respiration and sympathetic activity, following reversible blockade by vagal cooling. Changes in the degree of parasympathetic control on atrioventricular conduction will be assessed by changes in the PR interval. It is expected that not only will the dog model provide additional insight into the relationship between parasympathetic control and respiratory sinus arrhythmia, but it will also suggest a potentially clinically useful assessment of parasympathetic control by noninvasive means.